I have flown three times with syringes and needles for my daughter. Her medicine had to be kept cold, so I had a very small insulated bag (was made for baby bottles) and inside had the medication syringes (WITH LABELS ON!) wrapped in foil to keep it cold, and the needles in the same pouch. As I approached the x-ray machine I held it up and told them there were needles in it. One time they sent it through the machine and didn't do anything more. The other two times they unwrapped the foil, which I was expecting. As long as everything is properly labeled it shouldn't be a problem. Our needles were delivered in a plastic bag with contents (what gauge needle, etc) and name on it. I kept them in that. It was a little bulkier, but kept everything in proper order. You can always get a note from your dr if you are concerned. Worst case, I would think they could give the needles to the flight attendent to lock up and return to you at your destination.
Good luck!
Suzanne